Gas Discharge Tube Overview
End of Life
The gas discharge tubes are designed to withstand many impulses without destruction or loss of the initial characteristics (typical impulse tests are 10 times x 5kA impulses for each polarity).
On the other hand, a sustained very high current, i.e. 10A rms for 15 seconds, with simulate the dropping out of the AC power line onto a telecommunication line and will take the GDT immediately out of service.
If a fail-safe end of life is desired, i.e. short circuit that will report a fault to the end user when the line fault is detected, the gas discharge tube with the fail-safe feature (external short-circuit) should be selected.
Selecting a Gas Discharge Tube
The information required to properly select a surge protector for your application is the following:
- DC sparkover voltage (Volts)
- Impulse sparkover voltage (Volts)
- Discharge current capacity (kA)
- Insulation resistance (Gohms)
- Capacitance (pF)
- Mounting (Surface Mount, Standard Leads, Custom Leads, Holder)
- Packaging (Tape & Reel, Ammo pack)
Range of DC sparkover voltage available:
- Minimum 75V
- Average 230V
- High Voltage 500V
- Very High Voltage 1000 to 3000V
*Tolerance on the breakdown voltage is generally +/-20%
This depends on the properties of the gas, the volume and the material of the electrode plus its treatment. This is the major characteristic of the GDT and the one that distinguishes it from the other protection device, i.e. Varistors, Zener Diodes, etc… Typical value is 5 to 20kA with an 8/20us impulse for standard components. This is the value the gas discharge tube can withstand repeatedly (minimum 10 impulses) without the destruction or aleration of its basic specifications.
Impulse Sparkover Voltage
The sparkover voltage in the presence of a steep front (dV/dt = 1kV/us); the impulse sparkover voltage increases with the increasing dV/dt.
Insulation Resistance and Capacitance
These characteristics make the gas discharge tube practically invisible during normal operating conditions. The insulation resistance is very high (>10 Gohm) while the capacitance is very low (<1 pF).
Test Standards and installation recommendations for communication line surge protectors must comply with the following standards:
- UL497B : Protectors for Data Communications and Fire-Alarm Circuits
To be effective, the surge protector must be installed in accordance with the following principles.
- The ground point of the surge protector and of the protected equipment must be bonded.
- The protection is installed at the service entrance of the installation to divert impulse current as soon as possible.
- The surge protector must be installed in close proximity, less than 90 feet or 30 meters) to protected equipment. If this rule cannot be followed, secondary surge protectors must be installed near to the equipment